This invention relates to a ventilation apparatus for a motor vehicle comprising a member for defining an air channel, a multiply adjustable fan located within the air channel which is driven by an electrical motor, and a switching device coupled to the motor for multiple-step adjustments of operation of the fan, wherein is included performance resistors which are located within an effective ventilating range of the fan and which can be selectively switched by means of a step switch which is mechanically activated from a passenger compartment for controlling the electrical motor in steps.
Such apparatus can be found frequently in motor vehicles and are known from prior art. A publication "Kraftfahrtechnisches Taschenbuch" of the R. Bosch company shows in its 20th edition, pages 568 and 569, a circuit diagram of a motor vehicle, in sub-portion 7 of which is schematically disclosed a fan of a ventilation apparatus which is adjustable by means of series-connected resistors. Setting the ventilation apparatus to different ventilating levels adjusts an air volume thereby moved to requirements for the passenger compartment.
Such prior art ventilation apparatus normally has an electrical step switch located on a dashboard. For practical reasons, the step switch is directly coupled to an adjustment handle located inside the passenger compartment on the dashboard. The adjustment directions of the handle and the switch (that is, the arrangement of contacts of the step switch) are usually the same; for example, lengthwise or circular. An electrical motor of a fan of the ventilation apparatus can be coupled to a power source of the motor vehicle by means of the step switch. The fan which is driven by the electrical motor produces an air stream, or flow, in an air channel of the apparatus, thereby ventilating the passenger compartment. The fan can be adjusted stepwise through several switching steps of the step switch, wherein performance resistors with varying resistance values are placed in series with the electrical motor of the fan to provide several settings, or adjustment steps. A performance resistor arranged in series with the electrical motor reduces the drive power according to its value, thereby determining the airstream volume, or speed, for a particular ventilation level. The performance resistors are made of regular twisted resistor wire. Because the performance resistors develop a great deal of heat during operation, they are arranged a distance from the step switch, within effective ventilating range of the fan; that is, they extend into the air channel and are cooled by the air stream therein. In the prior art, performance resistors are arranged in a compact group of components which is often coated by a ceramic mass to prevent electrical sparks. The performance resistors are electrically coupled to the step switch and the electrical motor by means of a multi-wire cable.
For a 250 Watt electrical motor electrical currents of up to approximately 20 Amps can be drawn. Accordingly, the diameter of an individual conductor in a multi-wire connection cable is chosen to be 2.5 mm.sup.2. From experience, the temperature at the switch contact of the step switch increases by approximately 30.degree. C. at a current of 20A. The interior of a motor vehicle can reach 70.degree. C., for example in sunny weather, so that upon operation of the fan of the ventilation apparatus at full power for ventilating the interior, temperatures of approximately 100.degree. C. are reached at the switch contact of the step switch, that is, on the dashboard.
This circumstance and the great self-generation of heat by the performance resistors during operation leads to the known and frequently used segmentation of a stepwise adjustable ventilation apparatus so that a first component group of a switching device has a step switch, often with notches, on the dashboard, a second component group of the switching device frequently comprises several performance resistors arranged within the air channel, with a connecting element having a large-diameter multi-wire cable electrically coupling the first component group to the second component group, and a third component group being likewise electrically coupled to the step switch including a fan driven by the electrical motor.
Smoothly adjustable (non-step) ventilation apparatus are also known from the prior art. Such apparatus, which are in most cases electronically adjustable, are not related to this invention.
Recently, fans of ventilation apparatus have been required to deliver higher outputs, particularly when one or several filters arranged in air channels have become increasingly blocked with increasing life spans, thus hindering ventilation of passenger compartments because of increases in air-flow resistance. Such obstacles can be compensated at least partially by higher fan performance. In the past, fan motors of 250 Watt capacity have been used, but recently, fan motors with performance capacities of up to 500 Watts have come into use; and this may be further increased in the future. Thus, maximum operation currents are being increased, for example, to 40 Amps and more. As a result of such higher currents, the wire diameters of the multi-wire connection cables of the circuit apparatus must be increased to 4 mm.sup.2. Such a cable is substantially more expensive and more difficult to manipulate. Another result of higher currents is a rise in temperature at the step switch contacts, thus reducing the life span of the step switch and of neighboring, particularly electronic, components. These higher temperatures could, for example, be avoided by using a step switch with lower contact resistance, however, such a power switch often lacks a desired ease of adjustment and life span. It is an object of this invention to overcome these described problems and increased costs which have been brought about by increased performance demands by providing a ventilation apparatus of the type described above which is improved in a different manner.